This application claims the priority of German patent document 199 07 665.0, filed Feb. 23, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process and apparatus for carrying out a catalytic reaction, and for utilizing heat which is formed during the catalytic reaction.
It is sought, for example in the automotive sector (but also in other mobile applications) to minimize the mass, volume and costs of necessary components. To assure a problem-free cold start, the components which are required for the driving mode must have a low total mass and must be connected to one another by gas paths which are as short as possible.
A so-called heterogeneously catalysed reaction relates, for example, to the generation of hydrogen from hydrocarbons or alcohol, particularly methanol (methanol reforming), with a reaction mixture of hydrocarbons or alcohol and water being fed to a catalytic converter. Further examples of important reactions are the reduction of the carbon monoxide content, releasing carbon dioxide, in a so-called hydrogen shift reaction; the oxidation of carbon monoxide by feeding a CO-containing gas and an O2-containing gas to a catalytic converter; and the combustion of a combustible starting material with the addition of an O2-containing gas in a catalytic burner.
In hydrogen-powered vehicles, the hydrogen required is usually obtained on board the vehicle from hydrocarbons, for example methanol. The process of obtaining hydrogen from methanol is based on the overall reaction CH3OH+H2Oxe2x86x92CO2+3H2. In practice, to carry out a reaction of this nature, a reaction mixture comprising the hydrocarbon and water vapour is heated and guided past a suitable catalytic converter to generate the desired hydrogen, in a single-step or multistep reaction sequence. A device for two-stage methanol reforming of this nature is known, for example, from European Patent Document EP 0 687 648 A1. In the known device, the reaction mixture is fed to a first reactor, in which it is only attempted to achieve partial conversion of the methanol. After it has flowed through the first reactor, the gas mixture, which still contains a certain level of unconverted starting materials, is fed to a second reactor which is optimized with regard to residual conversion.
To ensure that the catalytic converter operates efficiently, it is necessary to vaporize the starting materials before they are fed to the reactor. For this purpose, it is customary to use heat exchangers of plate or tube design, since a large heat-transfer area is required.
Heat exchangers of this nature are also used to carry out further process steps during the catalytic reaction, for example to heat or cool gases which are formed or to supply heat to catalytically active materials or to correspondingly dissipate heat from these materials.
A drawback which has emerged in known devices, used for example to generate hydrogen from hydrocarbons, is that the large number of components required, particularly heat exchangers, results in devices which overall are of a large size. However, especially for applications in the mobile sector, it is sought to keep the mass, the volume (and therefore also the costs) of the components required as low as possible.
The object of the present invention is to provide a method and apparatus which can carry out a catalytic reaction using as few components as possible.
Another object of the invention is to provide a catalytic device whose components, individually and together, are of small size.
These and other objects and advantages are achieved by the method and apparatus for utilizing heat according to the invention, in which a vaporizer or vaporizer area, which is used to vaporize starting materials that are to be reacted, is formed as a single piece (or integrally) with a second area for at least partially carrying out the catalytic reaction or for further reaction of reaction products formed during the catalytic reaction and/or for at least partially cooling reaction products formed during the catalytic reaction. For example in the case of methanol reforming, the second area may be a CO oxidizer which is used for oxidizing carbon monoxide formed during the catalytic reaction, and/or as a cooling device for cooling reformate formed during the catalytic reaction. Thermally conductive communication between the two areas makes it possible substantially to dispense with, or to reduce considerably, the use of further heat exchangers.
Advantageously, a gas-impermeable, thermally conductive partition is formed between the first and second areas. This prevents starting materials which are to be vaporized from coming into contact with CO or reformate gases which are formed during the catalytic reaction.
In order to keep the thermal stresses in the partition plane as low as possible, the direction of flow on at least one side is designed to run perpendicular to the partition plane or wall. As a result, substantially isothermal conditions are established at the partition.
According to a preferred embodiment of the heat-transfer device according to the invention, the first and/or the second areas are made substantially from porous, (particularly metallic) materials of good thermal conductivity. This measure ensures that there is a large heat-exchanging area available. A porous structure further contributes to good heat transfer, due to the intensive mixing and turbulence of gases or fluids flowing through it.
Advantageously, catalytic material may be contained in at least one of the areas.
It has proven particularly advantageous for the respective areas, particularly in the area of the partition, to have regions which are of solid design. It is thus possible to effectively improve the thermal conductivity towards the partition wall or surface.
According to a particularly preferred embodiment of the heat-utilization device according to the invention, the first and the second areas are formed concentrically with respect to one another, the first area being arranged inside, and the second area outside, a gas-impermeable partition, which is of tubular design. A configuration of this nature makes it possible to ensure that reaction products which are to be cooled, for example, act uniformly, in the radial direction, on the second area even with a small space available.
Advantageously, in the area of the partition, passages parallel to the partition are formed in the second area; via these passages it is possible to dissipate gases acting on the second area. In this manner, it is possible effectively to dissipate reaction products which have been sufficiently cooled in the second area and have thus released their useful heat in order to heat the first area, without problems from the second area.
According to an advantageous embodiment, the device for carrying out a catalytic reaction generates hydrogen from a reaction mixture containing at least one hydrocarbon (particularly methanol) and water. The catalytic converter is designed as a reformer for carrying out a reforming step (particularly a partial oxidation of the reaction mixture to produce a reformate), and an element connected downstream of the catalytic converter comprises a CO oxidizer for oxidizing carbon monoxide formed in the reformer, and/or a reformate cooler for cooling the reformate which is generated in the reformer. The result is a hydrogen-generation device which is very small and can be used in particular in the automotive sector.
According to a preferred embodiment of the invention, a device for carrying out a catalytic reaction has a further element for preheating starting materials which are fed to the heater/vaporizer, and a further element for cooling reaction products which emerge from the element connected downstream of the catalytic converter. These further elements are in thermally conductive communication, so that it is possible to make effective, optimum use of the heat formed during a catalytic reaction.
It has proven particularly advantageous for the heater/vaporizer, the element connected downstream of the catalytic converter, and the catalytic converter to be formed integrally or as a single piece. This measure provides devices for carrying out a catalytic reaction which are especially small and eminently suitable for use in the automotive sector.
According to another preferred embodiment of the invention, a process is used to generate hydrogen from at least one hydrocarbon (particularly methanol), in which the vaporized reaction mixture is reformed (particularly, partially oxidized), in order to produce a reformate in a catalytic converter which is designed as a reformer. Carbon monoxide which is formed during the reforming step is fed to a CO oxidizer, and/or the reformate which is produced in the reformer is fed to a reformate cooler, with the heat which is released during the CO oxidation and/or the reformate cooling being used to assist with the vaporization of the at least one starting material. With a process of this nature, it is possible for heat formed during the cooling of the reformate or the CO oxidation to be used simply and efficiently to vaporize the reaction mixture which is to be reacted.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.